The power output of many multi-stage steam turbine systems is controlled by throttling the main flow of steam from a steam generator in order to reduce the pressure of steam at the high pressure turbine inlet. Steam turbines which utilize this throttling method are often referred to as full arc turbines becuase all steam inlet nozzle chambers are active at all load conditions. Full arc turbines are usually designed to accept exact steam conditions at a rated load in order to maximize efficiency. By admitting steam through all of the inlet nozzles, the pressure ratio across the inlet stage, e.g., the first control stage, in a full arc turbine remains essentially constant irrespective of the steam inlet pressure. As a result, the mechanical efficiency of power generation across the control stage may be optimized. However, as power is decreased in a full arc turbine there is an overall decline in efficiency, i.e., the ideal efficiency of the steam work cycle between the steam generator and the turbine output, because throttling reduces the energy available for performing work. Generally, the overall turbine efficiency, i.e., the actual efficiency, is a product of the ideal efficiency and the mechanical efficiency of the turbine.
More efficient control of turbine output than is achievable by the throttling method has been realized by the technique of dividing steam which enters the turbine inlet into isolated and individually controllable arcs of admission. In this method, known as partial arc admission, the number of active first stage nozzles is varied in response to load changes. Partial arc admission turbines have been favored over full arc turbines because a relatively high ideal efficiency is attainable by sequentially admitting steam through individual nozzle chambers with a minimum of throtlling, rather than by throttling the entire arc of admission. The benefits of this higher ideal efficiency are generally more advantageous than the optimum mechanical efficiency achievable across the control stage of full arc turbine designs. Overall, multi-stage steam turbine systems which use partial arc admission to vary power output operate with a higher actual efficiency than systems which throttle steam across a full arc of admission. However, partial arc admission systems in the past have been known to have certain disadvantages which limit the efficiency of work output across the control stage. Some of these limitations are due to unavoidable mechanical constraints, such as, for example, an unavoidable amount of windage and turbulence which occurs as rotating blades pass nozzle blade groups which are not admitting steam.
Furthermore, in a partial arc admission system the pressure drop (and therefore the pressure ratio) across the nozzle blade groups varies as steam is sequentially admitted through a greater number of valve chambers, the largest pressure drop occurring at the minimum valve point and the smallest pressure drop occurring at full admission. The thermodynamic efficiency, which is inversely proportional to the pressure differential across the control stage, is lowest at the minimum valve point and highest at full admission. Thus the control stage efficiency for partial arc turbines as well as full arc turbines decreases when power output drops below the rated load. However, given the variable pressure drops across the nozzles of a partial arc turbine, it is believed that certain design features commonly found in partial arc admission systems can be improved upon in order to increase the overall efficiency of a turbine. Because the control stage is an impulse stage wherein most of the pressure drop occurs across the stationary nozzles, a one percent improvement in nozzle efficiency will have four times the effect on control stage efficiency as a 1 percent improvement in the efficiency of the rotating blades. Turbine designs which provide even modest improvements in the performance of the control stage nozzles will significantly improve the actual efficiency of partial arc turbines. At their rated loads, even a 0.25 percent increase in the actual efficiency of a partial arc turbine can result in very large energy savings.